1. Field Of the Invention
The present invention relates to drilling into ground surfaces, such as in the areas of oil well drilling and gas drilling, e.g., notably from offshore platforms. In particular, the present invention involves an improved method of connecting composite tubular drill pipes to metal end fittings.
2. Description of the Background Art
Oil and gas well drilling in offshore environments has traditionally been dominated by vertical and near vertical well bore trajectories. By increasing the ability to depart from vertical trajectories, a larger field of oil or gas can be produced from a single operating platform. By drilling at an angle (i.e., a "sail angle") of, for example, 75.degree. from vertical, the weight of the drill string is used to push against the cutting bit. When the sail angle is increased toward being horizontal, the weight of the drill string develops frictional forces against the formation (e.g., against the ground) that resist the desired action of pushing ahead. As a result, the capacity to bore through the formation is lost.
During the drilling process, a heavy fluid (e.g., a "drilling mud") is pumped through the central bore of the drill pipe and through orifices in the drill bit. The fluid then returns through the annulus between the drill pipe and the bore hole. The drill pipe, which is typically steel and much heavier than the liquid or drilling mud, rests against the bottom side of the bore and significant friction develops between the drill pipe and the bore. By controlling the density of the composite drill pipe, a buoyant force can be used to reduce the effective weight of the drill pipe against the formation. As a result, the amount of friction can be reduced.
One known composite drill pipe is shown in U.S. Pat. No. 5,332,049. The '049 patent drill pipe enables boring deviated wells using a short radius of curvature with a limited reach. The '049 patent shows a method of transferring loads between a composite pipe body and a metal end fitting that consists of both an adhesive bond and mechanical pins. This method of attachment has a number of drawbacks. For example, by using a hard bonding and mechanical pins, if the fitting is compromised during drilling operations to the point of failure, there is no means to pull the drill string out of the bore hole. In addition, the overall strength of the fitting is limited. As one example, there is no effective method to ensure that the adhesive is loaded evenly.
As discussed in the '049 patent, the disclosure of which is incorporated herein by reference, extended reach drilling technology uses a drilling string consisting of a series of drill pipes. Typically, the drill pipes are made of steel, but other materials such as aluminum have been used. Drilling at a sail angle is used to balance the beneficial effect of gravity in pushing on the drilling bit and the creation of friction along the formation wall. However, as the bore progresses outward, it also moves deeper. As a result, the trajectory may not intersect the oil pay zones.
Another problem with the drill pipe in the '049 patent is that it is not appropriate for extended reach drilling. The disclosed drill pipe is designed to be rather flexible and, thus, has a rather low stability threshold. Friction buildup is not eliminated, and, eventually, the axial loads in the drill string build up to a point where the pipe loses stability and begins to buckle. Once the buckling commences, the boring force transmitted to the cutting bit (i.e., the "weight on bit", "WOB") is lost and no further drilling can be accomplished.
Furthermore, with the method of attachment of the '049 device--using a combination of bonding and pins to attach the metal end fitting to the composite pipe section--any failure during drilling from torsional loads will also destroy the axial load capability of the drill pipe and the pipe will separate.
Moreover, the overall capability of a bonded joint to transfer loads is limited due to the inherently different material properties between the composite and the steel end fitting. The steel end fitting cannot tolerate the strain range of the composite material. Because of this, the shear loading at the leading edge of composite to metal interface on a bonded fitting has a very high concentration of shear stress. As the axial load is increased, the magnitude of the local shear stress also increases. When the strength of the bond is exceeded in this local area, the bond fails locally and this failure quickly propagates along the entire length of the fitting. Attempts to increase the strength by adding an additional length to the interface are futile because the local concentration at the leading edge is minimally affected by an increased length and the over strength is relatively unimproved.